1. Field of the Invention
The present invention relates generally to apparatus and methods for determining the “true” depth of a well logging tool suspended from a cable inside a wellbore. This may be done while the cable is being raised or lowered inside a well bore. The method is based on processing of accelerometer measurements.
2. Description of the Related Art
Electric wireline well logging instruments are typically inserted into and withdrawn from wellbores by means of armored electrical cables. The logging instruments generate signals which are related to physical properties of the earth formations through which the wellbore is drilled. A record of the properties of the earth formations with respect to depth in the wellbore is generally made at the earth's surface by pulling the logging instrument out of the wellbore by reeling the cable onto a winch or similar spooling device, while simultaneously recording the signals generated by the logging instrument. The record of the measurements is thus made to correspond to the apparent depth within the wellbore at which the measurements were made by the logging instruments.
Measurement of the apparent depth of the instrument in the wellbore is typically performed with a calibrated wheel placed in frictional contact with the cable at the earth's surface. The calibrated wheel turns correspondingly with the amount of linear motion of the cable as the cable is moved into or out of the wellbore by the winch. The wheel can be rotationally coupled to a mechanical counter calibrated to indicate the length of cable moved past the wheel, or the wheel can be coupled to an electronic encoder connected to a computer or electronic counter to indicate and record the length of cable which has moved past the wheel. It is assumed that the length of cable extended past the wheel directly corresponds to depth of the instrument in the wellbore.
Calibrated wheels can accurately determine the total length of cable which has been spooled past the wheel into the wellbore, but the true depth of the instrument in the wellbore may not correspond exactly to the spooled length of cable because the cable is subject to change in its overall length as the tension on the cable varies. The tension on the cable is affected by things such as the total weight of the cable disposed within the wellbore, which can be as much as 500 pounds (227 kg) for each 1000 feet (305 m) of cable. Tension is also affected by the weight of the instrument when it is inserted into the wellbore, which weight can vary depending on instrument density (related to the weight of the instrument and how much of the instrument volume is enclosed air space) and the density of a fluid (“drilling mud” or “completion fluid”) which may fill the wellbore, and can also be affected by friction caused by movement of the instrument against the wall of the wellbore.
Friction is the least predictable of the causes of tension on the cable as it is moved into and out of the wellbore because the wall surface of the wellbore has an indeterminate degree of roughness and the earth formations penetrated by the wellbore have indeterminate frictional coefficients. The fluid which typically fills the wellbore can have indeterminate viscosity and lubricating properties at different depths within a particular wellbore, making determination of friction even more difficult.
It is frequently the case that the measurements made by the instrument are made at depths as much as ten feet or more different from the depth indicated by the calibrated wheel because of tension induced stretch in the cable. Various methods have been developed to correct the apparent depth measurements for changes in the stretch of the cable as caused by the previously described factors. U.S. Pat. No. 3,490,149 issued to Bowers, for example, describes using measurements made by accelerometers disposed in the logging instrument to calculate a change in axial position of the logging instrument, so that the cable length measurements made at the earth's surface can be corrected by using the calculated change in instrument position. U.S. Pat. No. 4,545,242 issued to Chan describes a more sophisticated method for using accelerometer measurements to determine a “correct” instrument position. Kalman filtering is used for the purpose. The function of the Kalman filter is changed when stick and pull is detected. During that interval less emphasis is placed on the uphole measurement than the downhole measurement.
U.S. Pat. No. 5,541,587 issued to Priest describes a method for determining correct depth of a well logging instrument using a combination of accelerometer measurements and a measurement of phase shift in an electrical signal passed through the logging cable, where the phase shift corresponds directly to the overall length of the logging cable. The phase shift measurement thus corresponds to the amount of stretch in the cable, this measurement being used to calculate instrument position where the accelerometer measurements are least effective and most erroneous, namely when the acceleration on the instrument is zero. U.S. Pat. No. 5,019,978 to Howard et al. discloses another method of depth determination in which Kalman filtering is used for depth determination. The software includes a parameter estimation routine for estimating the resonant frequency and the damping constant associated with the cable at different depths of the tool in the borehole.
The Kalman filter (or smoother) assumes that the noise in the measurements of acceleration and surface depth are assumed to zero mean and Gaussian. The method also assumes that the measurement noise is well characterized in terms of amplitude and correlation with the noise of other measurements. There is a need for a method of determination of tool depth in a borehole that makes a minimum of assumptions. The present invention satisfies this need.